This invention relates to submarine telecommunications systems employing electrical power feeding and in particular to branching units for optical fibre submarine cable systems.
Submarine cable systems originally were such as to connect two land terminals which were, for example, on the opposite sides of the Atlantic ocean. A later development involved having two land terminals on one side and a third on the other side, there being a main cable extending between the third land terminal and a branching unit (y interconnect) and spur cables extending from the branching unit to the other two terminals. Repeaters may be disposed in the main cable and the spur cables and power feed provision must thus be made. The repeaters can be powered by supplying current between a land terminal (terminal station) at one end of a branch (main or spur) cable and a distant earth (single end feeding) or between any two terminal stations (double end feeding). Branching units have previously been proposed whereby power feed of the repeaters in the main cable and one spur cable is by double end feeding, whereas that of the repeaters in the other spur cable is by single end feeding, the earth being provided by a sea earth cable extending from the branching unit. The branching unit may include relays by means of which the power feeding can be changed (switched) in the event of fault conditions in one of the branches, in order to isolate that branch whilst continuing to power the other branches. Such power feed switching can alternatively be achieved by use of remote signalling rather than relays.
Optical fibre submarine cable systems are now being proposed having a number of branching units, that is involving multiple landing points. The hitherto proposed branching units are unsuitable for use in such multiple branching unit systems since under certain switching conditions cathodic protection of the sea earth ground plate can be lost, in which case the ground plate will be eroded and may eventually disintegrate.
It is therefore, an object of the present invention to provide an alternative design of branching unit which can be used in multiple branching unit systems without giving rise to loss of cathodic protection.